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BEHAVIOR OF PRECAST PRESTRESSED CONCRETE HOLLOW CORE SLABS WITH DIFFERENT CONCRETE TOPPINGS

Abstract

Usage of precast prestressed concrete hollow core slabs has been extensively spread out in roofing and floor systems around the globe due to benefits of mass production and fast site construction. In this flooring system prestressed precast hollow core slabs are used together with or without a cast in place concrete topping, using concrete topping enhances the structural integrity of the system. This paper presents an experimental study conducted to investigate the effects of using different types of cast in place (CIP) concrete toppings on the flexural behavior of precast prestressed concrete hollow core slabs (HCS). Ten full scale HCS were prepared and classified into five groups. Group (1) includes two control HCS with no topping, Group (2) contains two HCS specimens with ordinary reinforced concrete topping connected to HCS using steel anchors with different patterns, Group (3) involves two HCS specimens with ordinary reinforced concrete topping without steel anchors, Group (4) comprises two HCS specimens with two different types of fibrous concrete topping, and finally Group (5) includes two HCS specimens with Ferrocement topping. All specimens were tested under line loading until failure. Cracking patterns, failure modes, cracking and ultimate failure moment capacities, and moment- deflection relationship have been illustrated in this study.

الخلاصة

إن استخدام البلاطات الخرسانية المجوفة سابقة الصب والإجهاد قد انتشر بصورة كبيرة حول العالم كإحدي نظم التغطيات  كنتيجة لمزاياه المتعددة, أهمها إمكانية الإنتاج الكمي وسرعة التشييد .في هذا النظام يتم استخدام هذه البلاطات معا ومن ثم صب طبقة رفيعة من الخرسانة فوقها كطبقة تغطية في بعض الأحيان لتحسين السلوك الإنشائي للنظام. هذا البحث هو نتاج دراسة معملية تم إجراؤها لتحديد تأثير إستخدام الأنواع المختلفة من الخرسانات كطبقات تغطية فوق البلاطات الخرسانية المجوفة سابقة الصب والإجهاد علي سلوكها في الإنحناء. تم إعداد عشرة  بلاطات  بالحجم الطبيعي وتصنيفها إلى خمس مجموعات.المجموعة الأولي و تشمل عدد 2 عينة مرجعية من البلاطات المجوفة بدون أي طبقات تغطية ، المجموعة الثانية وتحتوي علي عينتين من البلاطات المجوفة تم صب تغطية من الخرسانة المسلحة العادية فوقهم مع استخدام أشاير ربط بين التغطية الخرسانية والبلاطة المجوفة بأنماط مختلفة للأشاير، المجموعة الثالثة تتضمن عينتين من البلاطات المجوفة تم صب تغطية   من الخرسانة المسلحة العادية فوقهم بدون استخدام أشاير حديدية، المجموعة الرابعة وتشتمل علي عينتين من البلاطات المجوفة مدعمتين بطبقتين تغطية من نوعين مختلفين من  الخرسانات الليفية. المجموعة الخامسة والأخيرة وتحتوي علي عينيتين من البلاطات المجوفة مدعمتين بطبقتين من الفيروسيمنت كطبقة تغطية قليلة السماكة. تم اختبار جميع العينات حتي الإنهيارباستخدام أحمال خطية. في هذا البحث سوف يتم أستعراض أشكال الشروخ، وأنماط الإنهيار، وعزوم التشريخ وعزوم الإنهيار, كما سيتم أيضا أستعراض سهم الترخيم مقابل عزوم الإنحناء.

Keywords:  Hollow core slabs, Precast concrete, Prestressed concrete, Fibrous concrete, Ferrocement, full scale load tests, flexural strength, Concrete toppings, Failure modes, Steel anchors.

1. INTRODUCTION

Prestressed concrete hollow-core slabs (HCS) have been widely used throughout the world in concrete and steel structures [1],[2].  They were developed in the 1950s when long-line prestressing techniques evolved with the extrusion method, which allow production of inexpensive and easy-to-handle HCS [3]. A HCS can be defined as a precast, prestressed concrete member with continuous voids provided to reduce weight and, therefore, cost and, as a side benefit, to use for concealed electrical or mechanical runs[4],[5]. Primarily used in floors and roofs of residential, commercial, industrial and institutional buildings[6]. HCS have good fire resistance and sound insulation properties, and are capable of spanning long distances with relatively shallow depths. Common depths of prestressed hollow-core slabs range from 150 to 500mm which can achieve 20 m of span on roofs [7].

There are a number of situations where it may become necessary to increase the structural performance of concrete members either to incorporate the codes modifications or to allow for any change of use and the associated changes in superimposed loads [8],[9]. In recent years, the cement-base bonded overlay technique has been used to strengthen and enhance the structural performance of precast concrete slabs by adding a thin layer of cast in-situ reinforced concrete to the existing slab[10]. The primary purpose of this technique in HCS  is to overcome the camber caused by prestressing, which results in an uneven floor surface. Besides creation of a semi rigid diaphragm that connect hollow core slabs units together. This concrete layer consequently  improves the load-carrying capacity and stiffness of the slab by increasing its thickness[11],[12],[13],[14]. The main purpose of the current study focused on identifying the extent to which different types of  CIP concrete topping placed over surface of precast concrete hollow-core units improve the flexural behavior of the slabs.

2. EXPERIMENTAL PROGRAM

2.1. Test specimens

The aim of experimental work carried out in this study is to investigate the effect of using different types of structural concrete topping (screed) with and without connecting steel anchors on structural behavior of precast prestressed hollow core slabs subjected to static loads. For this purpose, ten full scale precast prestressed HCS were prepared. All specimens have a length of 4,100 mm, a depth of 160 mm and a width of 1,200 mm. Experimental program was done in two stages: first stage in which specimens were prepared, was carried out at Modern for Concrete factory and lab facility at Sadat city (local precast manufacturer), while loading and tests were at the second stage, which was developed at the Reinforced Concrete laboratory of the Civil Engineering Department of Menoufiya University. The test specimens classifications and naming convention are listed in table 1.

Group Name Legend Concrete topping Anchors No. of specimens

Group [1] HCS-C No topping No  Anchors 2

Group [2] HCS-RC-A

Ordinary

R.C. topping

[HCS-AF]

Ordinary R.C. topping [HCS-AS]

Anchors at Full span

Anchors at shear span

1

1

Group [3] HCS-RC Ordinary

R.C. topping No  Anchors 2

Group [4] HCS-FC

Steel fiber concrete topping[HCS-SFC]

Glassfiber concrete topping[ HCS-GFC]

No  Anchors

1

1

Group [5] HCS-FR Ferrocement topping No dowels 2

Geometry, cross section, and details of test groups are illustrated in Fig.1 to Fig.5.

2.2. MATERIAL PROPERTIES

2.2.1. Concrete materials

2.2.1. a. ordinary concrete

All the prestressed precast HCS  were manufactured in the precast factory. The average 28-day cube compressive strength of these slabs is 50 MPa. The unit weight of concrete used is 24 kN/m3. Concrete used for toppings in groups [2] and [3] are normal strength concrete with cube compressive strength of 25 MPa.

2.2.1. b. fibrous concrete

Two types of fibrous concrete are used as toppings, glass fiber and steel fiber concrete. 6 mm length measuring 13 microns in diameter monofilament glass fibers chopped from type E of glass are used to produce glass fiber concrete. The fibers are extremely fine, single filaments; fibers are coated with Silane based to improve initial dispersion and bond. 0.26% of cement weight was added to the mixture according to manufacturer bulletin.

Steel fiber concrete was prepared using high tensile undulated steel fibers made of cold drawn wire. Corrugated steel fibers are 55 mm in length and 0.8 mm diameter with Aspect ratio l/d: 68.75, fibers wire tensile strength is 1000 N/mm². 5.7% of cement weight was added to the mixture according to manufacturer data sheet.

2.2.1. ferrocement

The ferrocement laminates are reinforced using steel meshes locally produced and available in the market on commercial scale. Mesh grid size is 15x15 mm with 1 mm wire diameter. For concrete mortar, water/cement ratio used was 0.4, and the selected sand/cement ratio was 2.0.  

2.2.2. Reinforcement materials

Uncoated bright steel 7-wire P.C. strand (9.3 mm nominal diameter with nominal area of 52 mm2) low- relaxation strands were used. Average ultimate tensile strength and modulus of elasticity was found to be 1,860 MPa and 200 GPa; respectively.

Physical and mechanical properties for used materials are listed in table (2) and (3).

Topping Material Thickness Compressive strength of topping concrete (MPa)

Ordinary concrete 50 mm 25

Steel fiber concrete 50 mm 32

Glass fiber concrete 50 mm 36

Ferrocement 25 mm

25

Reinforcement  Material Yield stress(MPa) Ultimate strength Diameter

(mm) Mesh spacing

Prestressing strands ----- 1860 9.3 ----

10 mm high tensile welded mesh 360 520 10 200

Steel anchors 240 350 8 ----

Welded wire mesh 423 436 1 15

2.3. Test setup

The slabs are loaded using two line loads. The load was applied on the slab using two similar-sized steel I-sections in the loading frame, at approximately third points. The hollow-core slabs are supported on two stiff steel I-sections. The distance from the center line of the support to the end of HCS slab is 5 cm giving a clear span of 4.0 m. A 500 kN hydraulic jack was used to apply the load gradually. The complete detailed setup for testing of the hollow-core slabs is shown in Fig. 6.

2.3.1. Measuring Devices

The deflections at the center of the slabs and the center line of applied load were measured by three mechanical and electrical dial gauges (25 and 50 mm) attached to the bottom of the slabs. Demec points were attached to the concrete surface to measure concrete strain. The slabs were loaded gradually (5 kN per increment) to failure .The cracking load, failure progression and the cracks developed in the slab were monitored. The testing equipment and the test setup are shown in Fig.7 and Fig.8.

3. RESULTS AND DISCUSSION

3.1. Cracking and failure Loads

Figure 9 shows cracking and failure loads for all specimens. Cracking load was recorded upon emersion of first crack in slab soffit. Whereas failure load was   determined as slab resistance to load decreased significantly. Cracks propagation in all tested slabs followed the similar conventional flexural patterns in simply supported slabs. Initiation of flexural cracks on HCS specimens was observed to occur at middle third of the span directly below the applied load. As the loading increased, new cracks were formed on either side of the loading point. Control specimens HCS-C were first cracked on 80 kN with failure loads of 130 kN, and 135 kN respectively (average value for the two specimens is 132.5 kN). Applying traditional concrete topping in the second group HCS-RC lead to a direct enhancement in load carrying capacity and consequently improvement in flexural resistance of the slab. Cracking resistance increased to reach 95 kN with a gain 18.75 %.The increase in failure load for the same group were 11% as the failure load reached 150 kN. In the third group HCS-RCA which includes traditional concrete topping in addition to steel anchors, the improvement in cracking capacities were 25%, and 31.25% for specimens HCS-RCAS and HCS-RCAF respectively as the cracking load touches 100 kN, and 105 kN. Whilst raise in failure loads were 25.9%, and 40.7% for loads 170kN and 190 kN respectively. In the fourth group HCS-FC in which two types of fibrous concrete topping were used, the cracking loads were 90 kN and 95 kN for HCS-GFC and HCS-SFC with a total gain of 12.5% and 18.75% respectively. On the other hand, the failure loads for this group were 170 kN and 180 kN with 28.3% and 35.8%   enhancement in ultimate capacities for HCS-GFC and HCS-SFC respectively. In The last group which includes hollow core slabs with ferrocement topping, the cracking load was 85 kN for both HCS-FR1 and HCS-FR2 with a slight gain of 6.25%, on the contrary the failure loads reached 170 kN and 180 kN with 28.3% and 35.8%   enhancement in ultimate capacity. Figure 10 also shows cracking and failure moments for the all tested specimens.

3.2. Failure modes and crack patterns

The hollow core slabs may fail by many modes, flexural failure modes may be represented by concrete cracking at top due to prestress transfer, concrete cracking at bottom, and rupture of prestressing strands, crushing of concrete at top or excessive deflection under loads. While Shear failure modes may be seen by bond slip failure of strand, flexural shear failure, or web shear failure [2],[15] , see Fig. 11. In this study, flexural and shear modes were observed in the tested specimens as concrete cracking at bottom, flexural shear failure, and web shear failure. Crack patterns and some failure modes are presented in Fig.12 to Fig. 19.

3.3. Moment- deflection behavior

Observation of initial flexural cracks on HCS was occurred at slab mid span directly below the applied loads. When the cracks were visually detected for the first time, they usually extended from bottom into almost half of the depth of HCS. As the loading continued, new cracks appeared on either side of the loading point. Cracking was reflected through a change in slope of the load–deflection curve, after concrete cracking strands started resisting the applied load until specimen failure. A typical under-reinforced behavior for bending stresses was clearly noticed on moment deflection curves for tested specimens. The Moments versus mid-span deflection responses of the HCS are listed hereunder in the following subsections.

3.3.1. Effect of using traditional reinforced concrete toppings with steel anchors in the second group (HCS-RCA)

Figures 20 shows moment –mid span deflection behavior for the third group HCS-RCA in which traditional reinforced concrete toppings were used in addition to steel anchors located in the shear span for specimen HCS-RCAS and in full span in specimen HCS-RCAF. These results were compared to the control specimens. In pre-cracking and post cracking stage   deflection behavior shows excellent performance as there was full composite action till failure in HCS-RCAF, while specimen HCS-RCAS shows composite action only in pre-cracking stage. Both units reflected lower deformability and higher stiffness. In pre-cracking stage the highest deflection was 8.21 mm for HCS-C2 at a cracking moment of 106.7 kN.m, Specimens HCS-RCAS and HCS-RCAF show deflections of 6.72 and 10.58mms at cracking moments of 126.7 kN.m and 140 kN.m respectively. In the post cracking stage, the highest deflection was 57.78 mm for HCS-C2 at a failure moment of 180 kN.m, Specimens HCS-RCAS and HCS-RCAF show deflections of 85.17 and 71.5 mms at a failure moments of 226.7 kN.m and 253.3 kN.m respectively.

 

3.3.2. Effect of using traditional reinforced concrete toppings without anchors in the third group (HCS-RC)

Figure 21 shows moment –mid span deflection behavior for the third group HCS-RC in which traditional reinforced concrete toppings were used, results were compared to the control specimens. In pre-cracking and post cracking stage   deflection behavior shows lower deformability and higher stiffness. In pre-cracking stage the highest deflection was 8.21 mm for HCS-C2 at a cracking moment of 106.7 kN.m, Specimens HCS-RC1 and HCS-RC2 show deflections of 8.41 and 9.24mms respectively at cracking moments of 126.7  kN.m for both specimens. In the post cracking stage, the highest deflection was 57.78 mm for HCS-C2 at a failure moment of 180 kN.m, Specimens HCS-RC1 and HCS-RC2 show deflections of 72.48 and 72.4 mms respectively at a failure moment of 200 kN.m for both units.

3.3.3. Effect of using fibrous concrete toppings in the fourth group (HCS-FC)

Moment –mid span deflection behavior for the fourth group HCS-FC is shown in Fig. 22. Two types of Fibrous concrete toppings were used in this group. The results were compared to the control specimens. In pre-cracking and post cracking stage   deflection behavior shows lower deformability and higher stiffness. In pre-cracking stage the highest deflection was 8.21 mm for HCS-C2 at a cracking moment of 106.7 kN.m, Specimens HCS-GFC and HCS-SFC show deflections of 9.5 and 9.29 mms at cracking moments of 120 kN.m and 126.7 kN.m respectively. In the post cracking stage, the highest deflection was 57.78 mm for HCS-C2 at a failure moment of 180 kN.m, Specimens HCS-GFC and HCS-SFC show deflections of 95.7 and 127.5 mms at a failure moments of 226.7 kN.m and 253.3 kN.m respectively.

3.3.4. Effect of using ferrocement toppings in the fifth group (HCS-FR)

Ferrocement concrete toppings were used in this group as a thin topping to enhance flexural capacity of HCS units. Figure 23 shows moment –mid span deflection behavior for the fifth group HCS-FRC. The results of this group were compared to the control specimens. Both units reflected lower deformability and higher stiffness. In pre-cracking stage the highest deflection was 8.21 mm for HCS-C2 at a cracking moment of 106.7 kN.m, Specimens HCS-FR1 and HCS-FR2 show deflections of 13.55 and 13.88 mms at cracking moments of 113.3 kN.m and 140 kN.m respectively. In the post cracking stage, the highest deflection was 57.78 mm for HCS-C2 at a failure moment of 180 kN.m, Specimens HCS-FR1 and HCS-FR2 show deflections of 99.32 and 89.92 mms at a failure moments of 240 kN.m and 226.7 kN.m respectively.

Figures 24 shows comparison between moment deflection curves for second and third groups compared to control specimens. Concrete topping with steel anchors in the third group shows the best performance in stiffness, deformability, cracking resistance and failure load as shown below.

Comparison between moment deflection behavior for fourth and fifth groups is shown in Fig. 25 which shows better performance in deformability and higher cracking moment resistance for fibrous concrete .The two groups are equal in failure loads. Finally, moment deflection curves for all tested specimens is shown in Fig.26.

4. CONCLUSIONS

Full scale tests for ten precast prestressed HCS were carried out to investigate the extent to which a concrete topping placed over the precast concrete hollow-core units improves the flexural behavior. The following conclusions can be drawn as follows:

1. Adding  concrete topping to the HCS enhances flexural behavior directly under static loading, as the cracking moment resistance and failure moment increased by using concrete toppings.

2. Using steel anchors to connect the concrete toppings to HCS highly affected the performance of HCS units. They lead to less deformability and higher stiffness. Full composite action was occurred by using steel anchors in full span rather than shear span.

3. Replacing ordinary concrete topping with fibrous concrete topping without using any internal conventional reinforcement achieved higher cracking resistance, ultimate resistance with less deformability and higher stiffness.

4.  Reducing total slab thickness and load by using thin layer of ferrocement topping was also an effective technique, as it enhances the ultimate moment capacity considerably.

5. Using concrete overlay technique in strengthening of hollow core slabs was an easy effective technique as it improves the cracking moment capacity of HCS units by 6.25% to 31.25% and ultimate moment capacity by 13.2% to 43.3%.

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